It is well established that molecular orientation of plastics can provide enhanced mechanical, properties. Orientation is achieved by drawing or stretching the material under appropriate conditions of temperature. A number of methods have been proposed whereby this principle is applied to plastic pipes, in particular in order to enhance the burst strength under internal pressure by circumferential drawing.
United Kingdom Patent 1,382,580 describes the production of circumferentially oriented foamed PVC pipes by the expansion of a tubular feedstock in a cylindrical mould.
A difficulty arises in such methods in the provision, of a socket for the purpose of jointing of pipes together. Whilst there are many couplers and separate joining devices that might suffice for this purpose, the preferred system provides an integral enlarged bell on one end of the pipe, into which the other end or spigot may be inserted, with an appropriate sealing system, such as adhesive or rubber gasket. It is generally considered necessary that the socket section of such pipes should be strengthened in relation to the body, because of the increased diameter and hydrostatic stresses, and to cope with the variety of other loads to which the socket section may be subjected. A common procedure is to thicken the wall of the socket with respect to the wall of the pipe, proportionately to the diameters.
United Kingdom Patent 1,432,539 describes the production of an enlarged diameter socket portion by expansion of the feedstock into a female mould in such a way as to produce circumferential orientation. This method is unsatisfactory as a consequence of the reduced wall thickness of the socket portion, and subsequent patent applications by the same applicant have described alternative techniques. In Australian Patent Application 29086/77 the socket portion is formed from a portion with increased wall thickness, while in Australian Patent Application '29088/77 a sleeve is placed over the socket portion to counteract the loss of wall thickness.
The prior art proposals are cumbersome and not conducive to efficient manufacture. Feedstock thickening causes loss of dimensional control and requires exacting control of cutting of the extrusion to ensure correct positioning of the thickened section. Sleeving requires the production and fitting of the sleeve by a secondary process.
A further problem arises with oriented pipes if it is desired to provide fittings such as bend and tees with joints secured to the pipe to support axial thrust. Such devices can be utilised with un-oriented plastic pipes, as the axial strength of the pipe can accommodate the service loadings. For oriented pipes, axial strength should also desirably be increased by introducing axial orientation.
In the method described in U.S. Pat. No. 4,499,045, the method of U.K. 1,382,580 is modified by the use of a sleeve which moves progressively along the feedstock controlling its expansion. The friction between this sleeve and the feedstock produces longitudinal drawing of the tube, resulting in a biaxially oriented pipe. The method there described suffers from a lack of positive control over the degree of axial drawing of the feedstock, and offers no solution to the problem of providing a satisfactory socket by an economical process.
It is the purpose of the present invention to provide a method of manufacturing an oriented plastic pipe having on one end an integral socket with the necessary enhanced strength and wall thickness, without the need to pre- thicken the feedstock at the socket section. The invention is characterised by the use of controlled axial drawing of the feedstock, such that a differential axial draw ratio is applied to the socket and the body of the pipe to produce the desired relative thickness and properties.